Many physical processes are inherently exothermic, meaning that some energy previously present in another form is converted to heat by the process. While the generation of heat energy may be the desired outcome of such a process, as with a boiler installed to provide radiant heat to a building using a network of conduits which circulate hot water to radiators or a furnace used for the smelting of metals, in many other instances unwanted heat is produced as a byproduct of the primary process. One such example is that of the internal combustion engine of an automobile where the primary function is to provide motive force but where the generation of significant unwanted heat is unavoidable. Even in those processes where the generation of heat energy is desired, some degree of residual heat unavoidably escapes or remains which can be managed and/or dissipated. Whether generated intentionally or incidentally, this residual, or waste, heat represents that portion of the input energy which was not successfully applied to the primary function of the process in question. This wasted energy detracts from the performance, efficiency, and cost effectiveness of the system.
With respect to the internal combustion engine common to most automobiles, considerable waste heat energy is generated by the combustion of fuel and the friction of moving parts within the engine. Automobiles are equipped with extensive systems that transfer the heat energy away from the source locations and distribute that energy throughout a closed-loop recirculating system, which usually employs a water-based coolant medium flowing under pressure through jackets within the engine coupled to a radiator across which the imposition of forced air dissipates a portion of the undesired heat energy into the environment. This cooling system is managed to permit the engine to operate at the desired temperature, removing some but not all of the heat energy generated by the engine.
As a secondary function, a portion of the heat energy captured by the engine cooling system may be used to indirectly provide warm air as desired to the passenger compartment for the operator's comfort. This recaptured and re-tasked portion of the waste heat energy generated as a byproduct of the engine's primary function represents one familiar example of the beneficial use of waste heat.
Very large internal combustion engines are widely used in heavy industry in numerous applications. For example, General Electric's Jenbacher gas engine division produces a full range of engines with output power capabilities ranging from 250 kW to over 4,000 kW (by comparison, a typical mid-class automobile engine produces about 150 kW of usable output power). The Jenbacher engines can be powered by a variety of fuels, including but not limited to natural gas, biogas (such as provided by anaerobic digestion), and other combustible gasses including those from landfills, sewage, and coal mines. One common use of large combustion engines, such as the Jenbacher model 312 and 316 engines, is to co-locate them at a biogas generation facility. This consolidates, at one location, (i) the elimination of biodegradable waste products that release chemical energy in the form of combustible biogas and (ii) the capture and combustion of the biogas in large combustion engines to generate useful power.
These engines are frequently employed to drive electric power generators, converting the rotational mechanical energy from the energy of combustion into electrical energy. One such example of an anaerobic digestion system specifically designed for the generation of electric power from biogas is offered by Harvest Power of Waltham, Mass.
In operation, these engines generate tremendous amounts of waste heat energy that has historically been dissipated into the environment. In the case of the combined Jenbacher model 316 engine and generator system with a maximum electric power output of approximately 835 kW, approximately 460 kW of heat energy is lost in the exhaust gas (at an approximate temperature of 950° F.) and approximately another 570 kW is lost in the cooling system (with a typical jacket water coolant temperature of approximately 200° F.). From this data, it can be seen that less than half of the system's power output is in the desired form (in this case, electric power output from the system generator). Unless recaptured and repurposed, however, the portion of the input energy converted to heat is lost. In many prior art systems, this heat energy is lost and additional energy is required to cool the recirculating jacket water. The heat from exhaust gas generally escapes into the atmosphere, and the recirculating jacket water is cooled by an outboard apparatus (such as by large external condensing radiators driven by forced air sources), which consume additional electric power to function and further reduce the efficiency of the system.
Additionally, the dissipation of this waste heat energy into the environment can have deleterious effects. Localized heating may adversely affect local fauna and flora and can require additional power, either generated locally or purchased commercially, to provide additional or specialized cooling. Further, the noise generated by forced air cooling of the jacket water heat radiators can have undesirable secondary effects.
With regard to engines fueled by anaerobic-digestion-generated biofuel, a variety of techniques, including the use of electrical heating systems, have been employed to provide heat energy to anaerobic digestion processes necessary for relatively efficient generation of biogas by heated microorganisms. These systems consume considerable energy and therefore have an attendant cost of operation and maintenance. For example, the anaerobic digester heating systems offered by Walker Process Equipment of Aurora, Ill. produce hot water in excess of 160° F. using electric power with boilers fueled by biogas, natural gas, or fuel oil as input energy. In addition to the energy consumed to provide this hot water, additional electric energy must be consumed to manage the waste heat from this apparatus.
Waste heat energy systems employing the organic Rankine cycle (ORC) system have been developed and employed to recapture waste heat from sources such as the Jenbacher 312 and 316 combustion engines. One typical prior art ORC system for electric power generation from waste heat is depicted in FIG. 1. Heat exchanger 101 receives a flow of a heat exchange medium in a closed loop system heated by energy from a large internal combustion engine at port 106.
For example, this heat energy may be directly supplied from the combustion engine via the jacket water heated when cooling the combustion engine, or it may be coupled to the ORC system via an intermediate heat exchanger system installed proximate to the source of exhaust gas of one or more combustion engines. In either event, heated matter from the combustion engine or heat exchanger is pumped to port 106 or its dedicated equivalent. The heated matter flows through heat exchanger 101 and exits at port 107 after transferring a portion of its latent heat energy to the separate but thermally coupled closed loop ORC system which typically employs an organic refrigerant as a working fluid. Under pressure from the system pump 105, the heated working fluid, predominantly in a gaseous state, is applied to the input port of expander 102, which may be a positive displacement machine of various configurations, including but not limited to a twin screw expander or a turbine. Here, the heated and pressurized working fluid is allowed to expand within the device, and such expansion produces rotational kinetic energy that is operatively coupled to drive electrical generator 103 and produce electric power which then may be delivered to a local, isolated power grid or the commercial power grid. The expanded working fluid at the output port of the expander, which typically is a mixture of liquid and gaseous working fluid, is then delivered to condenser subsystem 104 where it is cooled until it has returned to a sufficiently liquid state.
The condenser subsystem sometimes includes an array of air-cooler radiators or another system of equivalent performance through which the working fluid is circulated until it reaches the desired temperature and state, at which point it is applied to the input of system pump 105. System pump 105 provides the motive force to pressurize the entire system and supply the liquid working fluid to heat exchanger 101, where it once again is heated by the energy supplied by the combustion engine waste heat and experiences a phase change to its gaseous state as the organic Rankine cycle repeats. The presence of working fluid throughout the closed loop system ensures that the process is continuous as long as sufficient heat energy is present at input port 106 to provide the requisite energy to heat the working fluid to the necessary temperature. See, for example, Langson U.S. Pat. No. 7,637,108 (“Power Compounder”) which is hereby incorporated by reference.
As a result of the transfer of waste heat energy from the combustion engine to the ORC system, these types of prior art ORC systems serve two functions. They convert this waste heat energy, which would otherwise be lost, into productive power and they simultaneously provide a beneficial, and sometimes a necessary, cooling or condensation function for the combustion engine. In turn, the ORC system's shaft output power has been used in a variety of ways, such as to drive an electric power generator or to provide mechanical power to the combustion engine, a pump, or some other mechanical apparatus.
ORC systems can extract as much useful heat energy as is practicable from one or more waste heat sources (often referred to as the “prime mover”), but owing to various physical limitations they cannot convert all available waste heat to mechanical or electric power via the expansion process discussed above. Similar in some respects to the cooling requirements of the prime mover, the ORC system requires post-expansion cooling (condensation) of its working fluid prior to repressurization of the working fluid by the system pump and delivery of the working fluid to the heat exchanger. The heat energy lost in this condensation process, however, represents wasted energy which detracts from the overall efficiency of the system.
Some prior art combined prime mover/ORC engine applications have utilized heat generated by the ORC condensation process in a conventional ORC system condenser while simultaneously providing power (electrical and/or mechanical) for various purposes. Combined heat and power (“CHP”) ORC systems have typically fulfilled a secondary purpose by using a portion of the heat energy from the prime mover and/or heat energy remaining in the post-expansion working fluid. FIG. 5 depicts a prior art ORC system including combustion engine heat energy output port 501 and condenser heat energy output port 502.
In one prior art ORC application, residual heat extracted from a dedicated ORC condenser during the cooling of post-expansion ORC working fluid at condenser heat energy output port 502 is used to provide domestic hot water, radiant heating, or both. This process requires the use of a conventional ORC condenser system well known in the art. The energy flow of such an application is depicted in the block diagram of FIG. 6. Here, a heat generating engine 601 is operatively coupled to electric generator 602 and provides waste heat energy 603 to the ORC system 604, which is operatively coupled to drive electric generator 605. Heat energy from the prime mover 601 is delivered to heat energy output port 501 and, in some prior art systems, is extracted to (i) a first heat energy input port 606 (such as for radiant heating) and (ii) a second heat energy input port 607 (such as for hot water heating). In those ORC systems known by the applicants, the utilization of residual heat from the post-expansion working fluid is intentionally extracted from the system but is not utilized for further system optimization of the prime mover or, for example, for heating a production material such as microorganisms to generate biofuel.